It has been desired that magnetic recording media, particularly for audio cassette tapes, should have better frequency response, and better reproduction capability with respect to an original recorded sound, particularly for audio cassette tapes used for recording music, and that such audio cassette tapes should have better runnability and durability with the increase of car stereo, cassette deck installed in radio set, etc.
On the other hand, a magnetic recording media for video cassette tapes has been used for high density recording by shortening the recording wavelength or narrowing track width. Therefore, magnetic recording media having higher output, higher signal/noise (S/N) ratio, and better reproduction capability with respect to original recorded images has been desired. Also, with the increasing use of portable video tape recorders (VTR), magnetic recording tapes for VTR having much better running durability than conventional tapes are desired, while simultaneously it is desired to reduce the total thickness of the tapes, for example, to 20 .mu.m or less.
In other words, audio tapes and video tapes having superior electromagnetic properties, running properties, and durability compared to conventional tapes, have been highly desired.
Various additives for the magnetic layer have been proposed, but such additives have not completely satisfied the desire for improvements as discussed above.
Research has been directed to a magnetic recording tape which is prepared by using ferromagnetic metal particles (i.e, a so-called "metal tape") to put such a product into practical use for video recording, because metal tape has higher recording density than the conventionally used iron oxide type recording tape. A system wherein a compact video tape recorder which uses a metal tape and which is associated with video camera has been proposed (Rajio Gijutsu (Radio Technique), a special number, May, 1981). In comparison with the VHS system or Beta system which have been commercially practiced, this proposed system is designed to have higher than two times the recording density, about 1/2 of diameter of the head cylinder, and about 1/2 of the relative speed between a head and a tape. Accordingly, the recording wavelength in the system would be about half that of the VHS or Beta system, and is assumed to have 1.mu. or less (about 0.6.mu.).
This proposed system is required to provide an image quality at least equal to or higher than that of the VHS system and the Beta system in recording and reproducing under these conditions. Therefore, if image quality of this system is evaluated under the same conditions as that of a VHS system and a Beta system, +6 dB or more of C/N ratio (that is, the ratio of an output or reproduced signals/modulation noise at recording FM carrying wave) higher than that of the VHS system or Beta system is required.
For a magnetic recording medium to satisfy the foregoing requirements, certain conditions must be met, but meeting such condition involves various problems.
For instance as the recording wavelength becomes shorter, it is required to increase coercive force (Hc) of a tape to some extent. Also, the surface of a magnetic recording layer must be made as smooth as possible to minimize a spacing loss between the magnetic head and the magnetic recording layer. For the purpose, the degree of dispersibility of the ferromagnetic particles must be increased and a calendering treatment must be carried out to make the surface of a magnetic recording layer smooth. These are also effective in decreasing noise.
Further, it is very effective and important to make the particle size of the ferromagnetic particles smaller in order to increase recording density of a magnetic recording medium. However, when the particle size is made smaller, problems occur, such as (1) that the dispersibility of the magnetic particles is deteriorated, whereby a magnetic layer having a smooth surface useful for short wavelength recording can not be obtained; (2) that durability of the magnetic recording layer is decreased, whereby head clogging reasily occurs in a still mode use of VTR; (3) that as the tape is running, output is decreased; and (4) that wear resistance is deteriorated, whereby the surface of the magnetic layer is scraped away by a magnetic head.
A magnetic layer is coated on a support, such as a polyester film, with a magnetic coating composition containing ferromagnetic particles, a binder, and organic solvents and other additives. The binder, which is the main component of the magnetic coating composition for the magnetic layer, must be selected from those that have excellent durability and wear-resistance. Various resins, such as a vinyl chloride-vinyl acetate resin, a cellulose derivative, and a polyurethane resin have been proposed as binders which meet the above requirements (U.S. Pat. Nos. 4,152,484 and 3,630,771). However, those binders do not have completely satisfactory characteristics.